1. Field of the Invention
This invention generally relates to a seismic data processing and in particular to a computationally efficient method for migrating the DMO stacking-velocity field, which is initially located at an unmigrated position, to its proper spatial location.
2. Discussion of the Prior Art
In the process of seismic exploration, arrays of receivers are emplaced along a grid consisting of multiple lines of profile in an area to be surveyed. An energy source insonifies the arrays by generating a wavefield that is reflected from sub-surface earth layers, to be detected by the receivers on the surface of the earth. The data signals from the respective receivers are combined or stacked in Common Mid Point (CMP) gathers. The geophysicist processes those gathers to image the depths and structural pattern of the sub-surface earth layers. The computer-processed data are displayed on suitable multi-trace cross sections or profiles analogous to geologic cross sections of the earth. There may be hundreds of thousands of individual data points originating from a given survey. For efficient computer utilization, the data must be compressed to a reasonable volume of data. The CMP stacking process is a common compression method.
The various methods for CMP stacking and dip migration are well known. For example see U.S. Pat. Nos. 4,742,497 and 4,943,950, assigned to the assignee of this invention and which are incorporated herein by reference; also see chapters 3 and 4 of "Seismic Data Processing" by Ozdogan Yilmaz, published by the Society of Exploration Geophysicists.
Unprocessed seismic reflection signals from a given earth layer, mapped as a function of travel time vs. offset distance from the source, form hyperbolae. A diffraction pattern from a point source in the earth such as a fault scarp also is hyperbolic. Prior to stacking, the hyperbolic envelopes of the reflected signals are rectified by application of correction-time differences that are computed from the stacking velocity. The stacking velocity may be constant or variable as a function of depth, depending on the geology of the region. The correction-time difference between the original reflection-time hyperbolic envelope and the rectified reflection time, for any given trace, is termed Normal Moveout (NMO). NMO is calculated from a stacking velocity appropriate to the two-way travel time to the reflection in question. By the same process, the diffractions are collapsed to their apices.
If a given earth layer has a slope or dip, the apparent stacking velocity increases with increasing dip angle. The calculated normal moveout derived from the apparent stacking velocity must therefore be compensated for dip as outlined in the '497 patent, by applying DMO (dip moveout).
The DMO stacking velocity originates from the unmigrated position of a given reflector beneath a selected CMP location. But a reflection from a dipping reflector, as perceived at a particular CMP location, does not lie directly beneath that location; it must be migrated up-dip laterally and to a shallower position in the cross section, to image properly the sub surface. Customarily, the observed DMO velocity was used for data migration after stack. But the DMO velocity is located at the unmigrated reflector position. For use as a migration velocity the DMO velocity itself should be migrated or repositioned prior to use in dip migration after stacking.
Steeply-dipping shallow events may interfere with flat-lying deeper events. Accordingly, the unmigrated DMO velocity determined for a particular event, in the presence of interfering or crossing events, will be multi-valued. Because of that problem, the resulting velocity spectrum will lack resolution.
This invention provides a computationally efficient method that uses conventional migration to migrate the DMO velocity to the proper spatial location thereby to provide a better estimate of the true sub-surface velocity and thus to migrate the seismic data with greater accuracy.